The invention relates to a gas friction pump circuit with a gas friction pump with rotor and stator, with a motor for driving the rotor, and with a circuit serving as a power supply for the motor.
The gas friction pumps include molecular and turbomolecular pumps, whose operation is described in detail in the textbook by Wutz/Adam/Walcher, "Theorie und Praxis der Vacuumtechnik," pages 202 ff. They have rotating and stationary components, which are so configured and spaced apart that the pulses transferred by the components to the gas molecules between them have a preferred direction. Gas friction pumps are high-vacuum pumps that have no tolerable back pressure, and therefore backing pumps must be connected to their discharge.
To determine whether a specific pressure has been reached in a vessel connected to a vacuum pump, it is known to use vacuum gauges. Vacuum gauges of this kind are relatively expensive. They are complicated to use, especially when all that is to be determined is whether or not a specific pressure limit has been reached. This is the case, for example, in the evacuation of hollow glass bodies for cathode ray tubes, which are sealed by fusion after a certain pressure is reached.
The present invention is addressed to the problem of creating a gas friction pump circuit of the kind described above, which in a simple manner will make it possible to know that a pressure limit has been reached by the pump.
This problem is solved in accordance with the invention by providing the circuit serving to supply power to the motor with electronic components by which the current drain of the motor can be recorded.
In the present invention use is made of the known fact that the current consumed by the motor of the gas friction pump is dependent on the pressure produced by the pump, and that this pressure is an indication of the pressure reached in the connected hollow body. In the study of this relationship I obtained the knowledge that there is a pressure range in which the friction power produced by the gas friction diminishes relatively rapidly with the reduction of pressure, while the friction power caused by the bearing friction varies but little. In this pressure range the gas friction becomes negligible, so that below the stated pressure range only the bearing friction remains to be overcome. I have found that these circumstances, in friction pumps of the same type, i.e., with a specific pump output, differ but little, so that it is possible to know, on the basis of the power drain, whether or not a specific pressure has been reached. It is therefore possible within the relatively great dependency of dI/dp, to establish a threshold which permits a repeatable pressure-related "go/no go" decision, where I represents current (in mA) and p represents pressure (in mbar). The means that are necessary for the establishment of such a switching threshold are substantially simpler than a complete vacuum gauge.
A desirable further development consists in indicating the current drain by a display. Such a display supplies relatively rough but reliable information on the pressure conditions that have been reached.